Remove default function template argument from the header.

GCC will treat the default function template arguments as a
compilation error if C++0x is not enabled.

This commit workaround the compilation error by moving the
SFINAE check to function argument instead of the template
argument.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@200523 91177308-0d34-0410-b5e6-96231b3b80d8